Both resistive ribbon thermal transfer printing and electroerosion printing are known in the art for providing acceptable resolution, good quality printing, especially of the type that is used in computer terminals and typewriters. Resistive ribbon thermal transfer printing is a type of thermal transfer printing in which a thin ribbon is used. The ribbon is generally comprised of either three or four layers, including a layer of fusible ink that is brought into contact with the receiving medium (such as paper), and a layer of electrically resistive material. In a variation, the resistive layer is thick enough to be the support layer, so that a separate support layer is not needed. A thin, electrically conductive layer is also optionally provided to serve as a current return.
In order to transfer ink from the fusible ink layer to the receiving medium, the layer of ink is brought into contact with the receiving surface. The ribbon is also contacted by an electrical power supply and selectively contacted by a thin printing stylus at those points opposite the receiving surface (paper) where it is desired to print. When current is applied via the thin printing stylus, it travels through the resistive layer and causes localized resistive heating which in turn melts a small volume of ink in the fusible ink layer. This melted ink is then transferred to the receiving medium to produce printing. Resistive ribbon thermal transfer printing is described in U.S. Pat. Nos. 3,744,611; 4,309,117; 4,400,100; 4,491,431; and 4,491,432.
The materials used in resistive printing ribbons are well known in the art. For example, the resistive layer is commonly a carbon or graphite-filled polymer, such as polycarbonate. The thin current return layer is a metal, such as Al. The thermally fusible inks are comprised of various resins having a colorant therein, and typically melt at about 100 degrees C. Printing currents of approximately 20-30 mA are used in the present, commercially available printers, such as those sold by IBM Corporation under the name QUIETWRITER.TM..
Electroerosion printing is also well known in the art. as exemplified by U.S. Pat. Nos. 3,786,518; 3,861,952; 4,339,758; and 4,086,853, Electroerosion printing is known as a technique which is suitable to make direct offset masters and direct negatives, Generally, the electroerosion recording medium is comprised of a support layer and a thin conductive layer. The support layer can be, for example, paper, polyesters such as Mylar.TM. etc., while the thin conductive layer is a metal, such as A1. In order to print, portions of the thin A1 layer are removed by an electric arc. To do so, a printing head comprising multiple styli, typically tungsten wire styli of diameters 0.3-0.5 mil, is swept across the electroerosion medium while maintaining good electrical contact between the styli tips and the aluminum layer. When an area is to be printed, a pulse is applied to the appropriate styli at the correct time, resulting in an arc between the energized styli and the aluminum layer. This arc is hot enough to cause local removal of the aluminum by disintegration, e.g., vaporization.
Practical electroerosion media require a base layer between the supporting substrate and the thin metal layer, as well as an overlayer on the thin metal layer. The base layer and the overlayer are used to prevent scratching of the aluminum layer in areas where no arc is applied, and to minimize head wear and fouling. Typically, the base layer is a hard layer consisting of hard particles embedded in a suitable binder, such as silica in a cross-linked cellulosic binder. The overlayer is typically a lubricating, protective overlayer comprised of a polymer including a solid lubricant, such as graphite in a cellulosic binder.
Each stylus of a commercial multi-stylus recording head used with resistive ribbon thermal transfer printing apparatus will have a diameter of about 1 to 4 mil, usually about one mil, particularly when used with the printer sold by IBM Corporation under the name QUIETWRITER.TM.. For high resolution printing, the size of a corresponding dot comprising ink transferred to a receiving substrate such as paper should be as close to the actual size of the stylus head as possible, that is about 1 mil in diameter. However, in practice, dot size is often as large as 4 mils in diameter using 1 mil styli. To a significant extent, the increase in dot size over stylus size is due to the thickness of the resistive layer in conventional self-supporting thermal transfer resistive ribbons, where the resistive layer, being a layer of 15 to 20 micron thick carbon-filled polycarbonate, also serves a support function. Considerable lateral heating of the resistive layer occurs, consequently increasing dot size. The 15 to 20 micron thick resistive layer has been considered necessary for maintenance of physical integrity of the resistive layer during the printing process, in the absence of a separate support. One approach considered to produce a resistive: thermal transfer ribbon providing higher resolution printing was to reduce the thickness of the resistive layer through a calendering operation whereby better carbon particle to particle contact would allow lowering the percent carbon loading, in turn resulting in a thinner resistive layer of higher mechanical strength. Calendering techniques for use with typewritter type ribbons are known, for example, see U.S. Pat. No. 1,830,559 to Pelton. Another approach was to provide a single resistive layer having an anistropic character so that the resistance is less in the direction of thermal transfer for printing than in the lateral direction. This approach is difficult to practice. Thus, thermal ribbon printing and production methods, and resistive ribbon products, are sought which will provide higher resolution printing when used with small diameter multi-stylus recording heads.